Television system



Aug. 7, 1945. K. R. wEND-r 2,381,238

TELEVIS ION SYSTEM Filed June 28, 1941 2 Smets-sheet 2 Snventor KarL R. wendt 'Bai i v Y Patented Aug. 7, 1945 UNITED STATES PATENTt OFFICE Karl R. Wendt, Audubon, N. J., assigner `to Radio Corporation of America, a corporation of Dela- Ware A Application June 28,1941, Serial No. 400,178

f 9 Claims. (Cl. 250-36) My invention relates to television receivers or,

the like and particularly to a method of and means for synchronizing cathode ray deflection circuits. The present application describes a simpliiied form of my invention' described and .claimed in my copending application Serial No.

400,177, led June 28, 1941, Patent No. 2,339,536 granted Jan. 18, 1944,

It has been the usual practice to synchronize the cathode ray deection or scanning at the re- ,ceiver withthe vscanning at the transmitter by transmitting to the receiver a mixture of picture signal and synchronizing pulses and by applying the received synchronizing pulses to a blockingA oscillator or thelike in the deiiection circuit for pulling the oscillator into step with the said pulses. This Vway of synchronizing a deecting circuit is satisfactory but it usually requires the use of a speedorfrequency control for the oscillator so that its free running oscillation frequency may be adjustedmanually to a frequency close to the rate at which the synchronizing pulses occur.

Also, a circuit so synchronized will fall out of l synchronism, immediatelyV upon the absence of synchronizingv pulses. v

An object of the present invention is to provide an'improved method of and means for lsynchronizing a cathode ray deecting circuit.

A further object of the invention is to provide a-cathode ray deecting circuit which will'minif mize the tendency to fall out of synchronism immediately upon the failure of synchronizing pulses. n A still further object of the invention is to provide a simplined cathode ray deecting circuit inV which the return line period is made to start, at or before the front edge of a synchronizing pulse.

In, practicing the invention, I utilize a simplified circuit for producing an oscillator frequency control voltage which changes in value withrany change in the phase relation 'of the incoming synchronizing pulses withrespect to the oscillator output. In a specic example, a diode is so connected in thegrid circuit of a blocking oscillator that the amount that the grid condenser of the blocking oscillator discharges duringa cycle of` operation depends upon the'phase relation of the applied synchronizing pulses and the oscillator 50 pulses. Thus, the oscillation' rate of the oscillator is held in a fixed phase relation to the synchronizing pulses. Y

The invention will be better..understood from vthe following description taken in connection with the accompanying drawingsin which and entitled Television sys- Fig. 1 is a block diagram of a television receiver embodying my invention,

Fig. 2 is a circuit diagram of the horizontal deection circuit of Fig. 1 designed in accordance with my invention,

Figs. 3a, 3b, 3c and 3d are curves which are referred to in explaining the invention, and

Fig.'4 is a circuit diagram of another embodiment of the invention.

Referring to Fig. 1, there is shown a televisionl receiver of the usual superheterodyne type comprising a first detector and tuning oscillator indicated at I0, an I.F. amplifier' II, a second detector I2,v a video amplifier I3 and\a cathode ray receiver tube I4. Y

- The vertical deflecting circuit I6 and the horizontal deiiecting circuit l(indicated by the bracket relation-of horizontal synchronizing pulses and y oscillator pulses.' Thus. the oscillator I9 is held in a fixed phase relation to the synchronizing pulses and the horizontal deectionY of the cathode ray of tube Il, likewise, is made to occur in the necessary fixed phase relation with respect to the horizontal deection at the transmitter.

One embodiment of the invention is shown in detail in Fig. 2 where parts corresponding to those in Fig. 1 are indicated by like reference chari acters.

Referring to Fig. 2, a separating circuit of the grid-leak biased type is shown at 26. The horizontal synchronizing pulses pass through a small capacity coupling condenserl 21 andare amplified by a vacuum tube 28. The resulting positive horizontal synchronizing pulses are applied through a blocking condenser 29 to the plate of a diode 3| having itsxcathode connected to the grid of the blocking oscillator I9. l

vReferring now to the oscillator I9, it is of the well known blocking oscillator design comprising an amplifler tube 32 having .a feed-back iron core at, I8 as is'well known in K,

transformer 33, a grid condenser 34 and a variable grid leak resistor 36.

The blocking rate or frequency of oscillation of a blocking oscillator depends upon the rate at which the blocking charge leaks 01T the grid condenser 34. In the usual circuit this rate, for a given grid condenser, is determined by the value of the grid leak resistor36. In the present circuit, it is determined alsoby the amount the grid condenser 34 can discharge through the diode 3|, this amount depending upon the, phase relation of synchronizing pulses and oscillator pulses.

It will be seen that in the absence of an oscillator pulse on the cathode of the diode 3|, the diode plate is held negative with respect to its cathode by the connection through a resistor 4I to a voltage divider 42. However, upon the occurrence of the negative pulse 43 (Fig. 3b) on the grid of the blocking oscillator, the cathode of diode 3| is driven negative sufficiently to make the diode conducting and the grid condenser 34 discharges through the diode a certain amount.A As indicated in volts on'the drawings (Figs. v3al and 3b) the amplitude of the blocking oscillator pulse is large compared with that of the synchronizing pulse. Y

The amount that condenser 34 discharges through diode 3| depends upon the instantaneous value. of the synchronizing `pulse voltage thatis being applied to the diode plate at the instant the negative oscillator pulse 43 (Fig. 3b) occurs. The circuit adjustment is such that the oscillator pulse 43 (Fig. 3b) and the synchronizing pulse 44 (Fig. 3a) occur in substantially the time relation shown in these two figures, that: is,the negative portion of the. pulse 43 falls on- `the sloping front of the pulse 44.

From the foregoing it will be apparent -that the amount of grid condenser discharge during each oscillator cycle, and therefore the oscillator frequency, will be determined by the phase relation of the synchronizing and oscillator pulses. If the oscillator tends to speed up, there will be lessvoltage across the diode electrodes and the grid condenser 34 will discharge less, thus holding back the oscillator tendency to speed up. If

the oscillator tends to slow down, the opposite action will take place.

The oscillator pulsesv may be `applied to the grid of a discharge tube 46 in the saw-tooth circuit 2|. A` saw-tooth voltage is produced in the usual way, a condenser `41 being vgradually charged through a plate resistor 48 and discharged rapidly, through the tube 46. The resulting saw-tooth wave is shown in Fig. 3c.

` The saw-tooth Wave from condenser 41 is ap-` starts substantially before the beginning of the` synchronizing pulse 44 whereby the return line period is terminated before picture signal for theV next s canningline is received. In a conventional circuit, the return line :c would startvat a later time with respect to the synchronizing pulse 44 as indicated in Fig. 3a, by the arrow'y and pieture signal might occur before'the return line ends.

A similar circuit is shown in Fig. 4 Where parts corresponding to those in Fig. 2 are indicated by the same reference characters. In this embodiment of the invention, the plate of the diode 3| is held negative with respect to its cathode between recurring pulses of diode current by means of a resistor 5l shunted by a condenser 52, the condenser 52 being charged by diode current pulses and being discharged slowly through resistor 5|. The circuit operation is essentially the same as described for Fig. 2.

It may be noted that, while the D.C. voltage von the grid of the blocking oscillator tube 32 is the plus B Voltage, the actual voltage on this grid is held suiciently negative by the charge on condenser 34 to block the tube 32 between oscillator pulses. v

. comprising aggrid capacitor and a. transformer coil in series, a unilateral conducting device condirection to permit electron current flow there through from said grid to said cathode and in parallel relation to said grid circuit with respect to said current flow for removing a charge from said capacitor, means for applying control pulses vto said device with the correct polarity to make it conducting, and 'means for maintaining said device in a non-conducting condition between the occurrencer of successive control pulses,

2. In combination, a blocking oscillator comprising a vacuum tube having a control grid and a plate and a cathode, and having coupled plate and grid circuits, said gridcircuit including a grid condenser and a grid leak resistor, a unilateral conducting device connected between said grid and said cathode in the direction to permit discharge of said grid condenser therethrough, means for applying control pulses to said device, and means for maintaining said device in a nonconducting condition between'the occurrence of successive control pulses.

3. In combination, a blocking oscillator comprising a vacuum tube having a control grid and a plate and a cathode, and having coupled plate and grid circuits,4 said grid circuit including a grid condenser and a. grid leakresistor, a unilateral conducting device connected between said grid and said cathode in the direction to permit discharge of said grid condenser therethrough, means for applying to said device control pulses each having a. sloping front edge, and means for maintaining said device in a non-conducting condition between the occurrence of successive control pulses whereby it becomes conducting during the occurrence of said front edge but only during the occurrence of a blocking oscillator v pulse of comparatively short duration.

.4. In combination, an oscillator of the blocking type which includes a condenser that yperiodically acquires a charge suicient to block the oscillator for a certain period, a discharge circuit for said condenser, aunilateral conducting device connected in said discharge circuit in series with biasing means to make it conductive only upon the occurrence of a blocking oscillator pulse, and means for impressing control pulses uponsaid device with the polarity to make it conducting for discharging said condenser by anamount which 4depends upon the phase rela.- tion of said blocking oscillator pulses and said control pulses.

5. In a cathode ray deection circuit, an `oscillator of the blocking type-Which includes a discharge circuit in series with biasing means to make it conductive only .upon the occurrence of a blocking oscillator pulse, and means for impressing synchronizing pulses upon said device with the polarity to make it conducting for discharging saidv condenser by an amount which 'f depends upon the phase relation of said blockythe occurrence of a blocking oscillator pulse, and

means for impressing synchronizing pulses upon said diode with the polarity to make it conducting for discharging said condenser by an amount which depends upon the phase relation of said blocking oscillator pulses and said synchronizing pulses.

7. In a cathode ray deection circuit, an oscillator of the blocking type which includes a condenser that periodically vacquires a charge sufficient to block the oscillator for a certainy period,v a discharge circuit for said condenser, a unilateral conducting device connected in said discharge circuit in series with biasing means so that it isconductive only upon the occurrence of a blocking oscillator pulse, and means for impressing synchronizing pulses having a sloping edge upon said device with a polarity that'makes it conducting to dischargev said condenser by an 'amount which depends upon the phase relation of said blocking oscillator pulses and said synchronizing pulses.

8. In combination, a blocking oscillator comprising a vacuum tube having a control grid, a plate and a cathode, and having coupled plate and grid circuits, said grid circuit including a grid condenser, a grid leak resistor, through Which said condenser may discharge, a second discharge circuit for said'grid vcondenser which comprises a circuit including a diode having a cathode connected to said control grid and having an anode connected to said first cathode, said diode circuit vbeing in parallel relation to said grid circuit which includes said grid condenser, means included in the diode connections for maintaining it in nonconducting condition in the absence of pulses applied thereto, and means for so applying control pulses to said diode that the amount of discharge of said grid condenser depends upon the phase relation of said blocking oscillator pulses and said control pulses.

9. In a cathode ray deflection circuit, a blocking oscillator comprising a vacuum tube having a control grid, a plate and a cathode, andv having coupled plate and grid circuits, said grid circuit including a grid. condenser, a grid leak resistor through which said condenser may discharge, a second discharge circuit for said grid condenser which comprises a diode having a cathode Vconnected to said control grid and having an anodeconnectedto said first cathode, means included in the diode connections for maintaining it in nonconducting conditionin the absence of pulses applied thereto, and means for applying positive synchronizing pulses having sloping sides to the anode of said diode whereby the amount of discharge of said grid condenser depends upon the phase relation of said blocking oscillator pulses and said synchronizing pulses.

KARL R. WENDI'. 

